Question

A motor boat going downstream overcomes a float at a point $A$. $60$ minutes later it turns and after some time passes the float at a distance of $12km$ from the point $A$. The velocity of the stream is (assuming constant velocity for the boat in still water)

• A. $6km{h}^{-1}$
• B. $3km{h}^{-1}$
• C. $4km{h}^{-1}$
• D. $2km{h}^{-1}$

Answer 1

The correct option is B $3km{h}^{-1}$

Let the speed of stream be v and speed of motorboat be u.
Now distance moved by boat in 1 hour $=(u+v)\times 1=u+v$
Total time is$=6/v$
Speed of boat when it turns back$=u-v$
=>$u+v-6=(u-v)(6/v-1)$
Solving for v we get,

$v=3km/hr$